Giant rat

Dude, this is seriously the “rat, giant (Sumatran)” from the Monster Manual.

Here’s some trivia – the (Sumatran) part of the entry header comes from a throwaway mention in one of Sir Arthur Conan Doyle’s “Sherlock Holmes” stories. In “The Adventure of the Sussex Vampire”, reference is made to “…the giant rat of Sumatra, a story for which the world is not yet prepared.” I’m certain Gygax was aware of the line, and that made its way into the header.